Evolution – basic principles & applications to health and illness
Adapted by Prof. Henry O'Connell from chapters 2 & 3 in his book "Evolution and Psychiatry: Clinical Cases" - Edited by Dr Gurjot Brar
"Evolution and Psychiatry: Clinical Cases" is available here
For this instalment, we go back to the basics of evolution. In the first part of this article, we outline general evolutionary principles, involving reproduction, mutation and selection. We have gone from Dawkins back to Darwin and Wallace themselves, as their principles are as relevant now as they were when first described over a century and a half ago.
In the second part of this article, we move on to the application of evolutionary principles to the study of health and illness in our species. Key principles from the work of Randolph Nesse are described, including the slow nature of evolution (leading to health problems relating to environmental mismatch and evolutionary ‘arms races’ with microorganisms), the blind and imperfect nature of evolution (leading to constraints, vestiges and trade-offs) and the fact that evolution is not interested in our happiness or well-being, but in the propagation of our genes. We end with a cautionary note against viewing diseases and disorders as being advantageous or adaptive.
The principles outlined in this article will be continually referred back to in future instalments, on specific psychiatric conditions.
Evolution – a very brief introduction
‘We are surrounded by endless forms, most beautiful and most wonderful, and it is no accident, but the direct consequence of evolution by non-random natural selection – the only game in town, the greatest show on Earth’.
Richard Dawkins. ‘The Greatest Show on Earth’ (2009)
Richard Dawkins has been one of the world’s leading evolutionary scientists for the past four decades, doing more during that time than anyone else to popularise evolution and bring the ideas to a wider, general audience. He followed up his ground-breaking 1976 classic ‘The Selfish Gene’ with a dozen more books on the topic of evolution. By nature a rigorous and systematic thinker and communicator, his above referenced quotation is effusive even by his standards. However, it reflects his unbridled enthusiasm and passion for evolutionary science, something I see in everyone who gets involved in the field, from newbies who are just dipping their toes in the water through to career-long evolutionists. The quotation is also reminiscent of Darwin’s ending to ‘On the Origin of Species’ (1859), when he refers to the ‘endless forms’ arising from ‘simple beginnings’. And Darwin, like Dawkins, was also an evolutionist who wrote for both a scientific and general audience.
In this article I will provide an outline of some of the key principles of evolution and define the main terms and concepts that will recur in future instalments. Regarding the issue of ‘proof’ of evolution, I will take it that you already accept the boundless evidence for this ‘theory’ from such disparate fields as palaeontology, embryology and molecular genetics. I will assume that you accept that we are all products of an evolutionary process that began with atoms, proteins and subsequently unicellular organisms that mutated, evolved and developed over several billion years to give rise to a sentient organism so complicated and flawed as yourself. If you are not yet convinced of the evidence, then I would suggest you put this book down and first read a book such as ‘The Greatest Show on Earth: the evidence for evolution’ (2009), by the aforementioned Professor Dawkins.
So to begin with the very basics, three key conditions must be fulfilled in order for evolution to take place:
An organism must be capable of reproduction. In the case of humans and other mammals, reproduction takes place by sexual means, with asexual reproduction occurring in microorganisms.
Secondly, that process of reproduction results in offspring that are different (even very slightly) from the parent and from any siblings. These differences arise because of the random mixing of genetic material from parents (in the case of sexual reproduction) and random changes or mutations in genetic material. These latter mutations may confer benefits to the offspring, may cause problems or may have neutral or no effects.
Finally, the offspring are ‘tested’ and ‘selected’ by their environmental conditions, with only those who are most suited (the ‘fittest’) managing to survive and reproduce themselves. Then the whole process starts again. It’s as simple and profound as that. The process can be applied to all living things, from viruses to chimpanzees. Evolutionary principles have even been applied to the development, ‘survival’ and spread of ideas with Dawkins using the idea of ‘memes’.
Selection
Selection is the second part of the above outlined evolutionary process and there are three main ways in which it can be mediated:
‘Artificial’ Selection
‘Natural’ Selection
‘Sexual’ Selection
As seen in our capacity to breed certain strains of animals or plants, selection can be ‘artificial’. One of the clearest everyday examples of this process is the domesticated dog. Starting with the wild wolf ancestor, we have over multiple generations selected dogs for breeding based on their temperament and appearance, along with their suitability for and usefulness to humanity. Thus, the modern friendly family dog is almost incomparable in terms of temperament and appearance from its wild ancestors of just a few thousand years ago.
Darwin himself was of course aware of this process of artificial selection in breeding animals such as pigeons. Furthermore, his knowledge of the power of human or ‘artificial’ selection was a key driver in developing his thinking on evolution generally.
Based on his knowledge of the power of ‘artificial’ human selection for breeding animals, Darwin worked slowly but steadily towards his discovery of ‘natural’ selection, whereby organisms are pitted against their environment, with the environment ‘selecting’ only the fittest organisms to survive and reproduce. Famously, he noted on his voyage aboard the HMS Beagle that the finch populations of the various islands of the Galapagos archipelago differed slightly from each other. Over subsequent years it occurred to Darwin that the various finch populations had a common origin but, when they spread to the various islands and started to mutate and evolve as independent island groups, they gradually drifted apart from each other in terms of physical characteristics and, had Darwin known about the concept at the time, their genetic make-up.
Added to these ‘internal’ genetic changes within the birds were the specific ‘external’ environmental challenges of the different islands, which ‘selected’ slightly different breeds with different characteristics on different islands. Add to this ‘recipe’ an unimaginably long time period (e.g. several million) and the process of ‘speciation’ starts to come into view more clearly. Darwin’s genius was that, despite any evidence at the time for the existence of genes and only a developing knowledge of the true extent of the length of time life was in existence, he realised that what he was witnessing was a brief snapshot of the first stages in the process of ‘speciation’, i.e. organisms gradually drifting apart in terms of their morphology, behaviour and genetics over time, to the extent that they would ultimately be unable to reproduce with birds from neighbouring islands, thus constituting separate species.
“An example of speciation is the Galápagos finch. Different species of these birds live on different islands in the Galápagos archipelago, located in the Pacific Ocean off South America. The finches are isolated from one another by the ocean. Over millions of years, each species of finch developed a unique beak that is especially adapted to the kinds of food it eats. Some finches have large, blunt beaks that can crack the hard shells of nuts and seeds. Other finches have long, thin beaks that can probe into cactus flowers without the bird being poked by the cactus spines. Still other finches have medium-size beaks that can catch and grasp insects. Because they are isolated, the birds don’t breed with one another and have therefore developed into unique species with unique characteristics. This is called allopatric speciation.”
Finally, in sexual reproduction, only the ‘fittest’ organisms (again in terms of e.g. temperament and physical qualities) are selected by those of the opposite gender for reproduction, thus leading to ‘sexual selection’. Another example from Darwin’s work is illustrative here. He was puzzled for a long time at how a male peacock could apparently waste energy in developing such colourful plumage, a characteristic that seemed to confer no functional advantage and that could even put the bird at higher risk of being picked off by predators.
“The sight of a feather in a peacock's tail, whenever I gaze at it, makes me sick,"
Charles Darwin. 1860.
The question of the peacock’s tail bothered Darwin for a long time until he discovered that there was indeed much method to the madness of these showy birds - the peahens chose to mate with the most brightly coloured males, thus ‘selecting’ the capacity for brightly coloured tails in future generations. Of course, the capacity to grow a brightly coloured tail probably indicates that the peacock is strong and healthy in other ways, so the coloured tail is also likely a proxy marker for a healthy mate.
A fundamental principle of evolution is that there is no intentionality in the process, i.e. organisms change from generation to generation and, depending on the prevailing environment against which they are pitted, they either survive, thrive and reproduce or not. Richard Dawkins has famously broken this down to the gene level, whereby organisms are merely carriers for genetic material, with the genetic material striving to replicate and see copies of itself in future generations. Through countless generations over unimaginable aeons of time, this random process leads from simple unicellular microorganisms through to the exquisitely complex, such as ourselves. It is an understandable human error of thinking to assume that complex organisms such as ourselves were ‘designed’ with a purpose by some higher or guiding power. However, the evolutionary perspective argues quite the contrary: our current state and status as a species was in fact precisely arrived at through tiny changes across countless generations and over a mind bending 4.6 billion years or so.
These basic principles of evolution, i.e. the changeable nature of individuals and species from generation to generation and the selection of the fittest organisms by artificial, natural and sexual means, combined with a lack of any intentionality or ‘intelligent design’ all seem obvious now, but the ideas were quite revolutionary when first presented in a coherent way by Charles Darwin in 1859 with his ‘On the Origin of Species’. The scientific establishment took some time to acknowledge and accept Darwin’s great discovery. And Darwin’s reluctance in publishing his findings was doubtless influenced by the prevailing level of religious belief and acceptance of creationism in the Britain of his day. Darwin had arrived at his conclusions following decades of painstaking research and prior to his work, even the idea of ‘transmutation’ or change in species from generation to generation was not widely accepted.
Darwin was thus slow to rush to conclusions and publish his findings. Although he gathered enough material for a lifetime of research during his five-year voyage on the HMS Beagle from 1831-1836, he went on to spend two more decades performing painstaking self-directed research from his home at Down House on topics as diverse as barnacles, various plant species and the aforementioned pigeons. It was not until he received a letter from a young Naturalist called Alfred Russel Wallace that Darwin felt compelled to publish the findings of his life’s work. At the time, Wallace had been travelling through Southeast Asia collecting specimens and documenting his own findings and reflections. Independent of Darwin, Wallace developed his own remarkably similar ideas on evolution and shared them with Darwin. Ultimately, their findings were jointly presented at the Linnaean Society in July 1858, findings that would go on to rock the scientific establishment and wider society.
Evolution is a fundamental bedrock of modern biology, providing the ultimate overarching framework for how we view life, our origins and the very nature of existence. And yet, evolution seems to be unheard of in the world of clinical medicine. There are many great minds throughout the world working on correcting this, including those are Randolph Nesse, Martin Brüne, Dan Stein and Alfonso Troisi and others such as Riadh Abed, Paul St. John Smith, Derek Tracy and Adam Hunt in Darwin’s own home country.
There are many potential reasons for evolution being largely omitted from medical education, clinical practice and research strategy. The power of religion and creationism is a significant barrier in some parts of the world. But even in largely secular countries in Europe, evolution is also ignored in clinical medicine. I can only surmise that busy and pragmatic clinicians see no role for evolution informing their everyday clinical practice – the ideas are interesting but on first glance seem to lack any practical use. Those who develop curricula for undergraduate and postgraduate medical education programmes see evolution as yet another topic that is just one too many to squeeze into already gigantic curricula. Funders of research, such as pharmaceutical companies, may find it difficult to see how research in this area can lead to the development of specific and marketable products. Finally, and not insignificantly, there may be reluctance within the medical profession and within wider society to embrace evolution because of previous widespread abuses due to malignant and erroneous interpretations, as seen so horrifically and tragically in Nazi Germany.
As a doctor and as someone who thinks deeply about the human condition generally, I find it hard to see how clinical medicine and psychiatry can be truly scientific if the core platform for biology and its one overarching explanatory model continues to be ignored. As a clinician I can also see how embracing an evolutionary perspective can add extra layers of understanding for doctors, new therapeutic and research approaches and ultimately better care for our patients.
Evolution and Illness – 7 key principles
I will now attempt to interweave the worlds of clinical medicine and evolution, describing how the evolutionary perspective is the essential underpinning framework for clinical science, as it is for all biology.
I am drawing primarily on the work of Randolph Nesse, who has been the world’s leading evolutionary physician for the past three decades. Nesse has been an inspiration to me for several years and I have had his valued support over the past three years in the formation of our College of Psychiatrists of Ireland Evolution and Psychiatry Special Interest Group along with the privilege of co-authoring an article with him (O’Connell et al, 2020). Nesse’s 2018 ‘Good Reasons for Bad Feelings’ was a very welcome publication for me and all clinicians with an evolutionary perspective, the book serving as an accessible and thoroughly enjoyable distillation of his research and writing going back to the early 1990s.
The principles outlined here are taken from ‘Good Reasons for Bad Feelings’ and other writings of Randolph Nesse. These principles are deceptively simple at first glance so I would encourage readers to think about them deeply, one at a time, and reflect on how they can be applied in everyday practice to improve the treatments we can provide for our patients. Clinicians from all professional backgrounds and at all stages of career development are likely to come up with at least a few conditions and situations where the application of these principles is likely to be appropriate, interesting and, most importantly, useful for their patients.
To avoid falling into the trap of ‘just so’ thinking I would also advise readers to think about practical, testable research questions that can be applied to each principle in a clinical or laboratory setting.
The first six principles are outline below and are taken directly from ‘Good Reasons for Bad Feelings’, with my own further expansion in the text that follows. I have also added (again taken from Nesse) an additional principle, relating to the importance of not falling into another trap potentially associated with the evolutionary perspective, i.e. the error of Viewing Diseases as Adaptations (VDAA).
Mismatch: our bodies are unprepared to cope with modern environments.
Infection: bacteria and viruses evolve faster than we do.
Constraints: there are some things that natural selection just can’t do.
Trade-offs: everything in the body has advantages and disadvantages.
Reproduction: natural selection maximizes reproduction, not health.
Defensive responses: responses such as pain and anxiety are useful in the face of threats.
Six Evolutionary Reasons why Bodies/Minds are Vulnerable to Disease
Randolph Nesse (2018)
1. Mismatch: our bodies are unprepared to cope with modern environments.
Our last common ancestor with modern Chimpanzees lived approximately 7 million years ago. Perhaps twenty different hominid or human like species have since evolved along that line of descent and gone extinct, with modern humans arriving on the scene just 150-200,000 years ago.
For the first 95% of our time (at least) as a species, we lived in small groups of closely related individuals numbering up to 50-150 at most, managing to survive on hunting and gathering. It was during this period that our current human faculties and characteristics were shaped, honed and selected by nature. It is only in the last 10,000 years that most of our species changed to a settler lifestyle, beginning with more organised forms of farming that ultimately led to the establishment of villages, towns and cities. Fast forward another 9,800 years and we have the industrial revolution, the establishment of large urban conurbations and, finally, the post-industrial landscape (at least in the so-called western world) of today.
Bearing in mind these admittedly hard to fathom timescales, it follows that much of what we are evolved and prepared to deal with as a species (i.e. subsistence living in small hunter-gatherer groups of closely related individuals with the ever present threat of death from starvation, conflict with other humans or predation), is no longer relevant.
In fact, some of the faculties and behaviours that evolved in our pre-agricultural past may now be pathologically or dysfunctionally ‘mismatched’ to our modern environments and, because evolutionary change is infinitesimally slow in comparison to the lightning fast cultural and technological changes of the past few hundred years in particular, many of our modern maladies can be seen as the struggles of a ‘stone age’ species striving to survive in a futuristic and highly ‘unnatural’ world.
A good concrete example of evolutionary mismatch with which to begin is that of our tastes and appetites for particular foods. In the pre-agricultural environment of our ancestors, highly calorific sugary and fatty food would have been in short supply. Those who had a taste for such food types and a propensity to ‘binge’ on them when they became available are likely to have had survival advantages over others. Contrastingly, in many modern environments, access to such foods is easy and cheap and supply is limitless. Therefore, our ‘stone age’ appetites are likely to drive many of us to excess consumption with associated health problems including obesity, diabetes and some cancer types.
Likewise, taking from psychiatry, certain anxiety states and disorders could be used as examples of illness arising due environmental mismatch. For example, it is plausible to consider that background levels of anxiety and agoraphobia may have had protective roles in the relatively more dangerous environment of our ancestors whereas in our generally safer and more comfortable modern environments such symptoms are perceived as being due to illness.
2. Infection: bacteria and viruses evolve faster than we do
Bearing in mind that reproduction is the engine that drives the pace of evolutionary change, it follows that rapidly replicating microorganisms are always several steps ahead of large multicellular organisms such as ourselves in the evolutionary race. So while our immune systems respond slowly to new microbial threats with tiny, if any, changes from generation to generation, our microorganism adversaries (and allies) can produce hundreds of thousands of generations during the timescale of a single human lifetime. And with each microorganism generation there is the chance that new and highly virulent strains will emerge that have the power to cause untold suffering and stall human civilization in its tracks, as seen so dramatically with the COVID-19 pandemic and countless other pandemics in human history, from the multiple waves of bubonic plague of the middle ages onwards, through to the ‘Spanish Flu’ pandemic of 1918-1919 that famously took more lives than the preceding World War I.
The rapid human sociocultural changes of the past 10,000 years have added further fuel to this dangerous human-microorganism mismatch. It has been argued (Rook at al, 2014) that human stress and innate antimicrobial response systems such as pyrexia and certain acute phase reactants evolved slowly and steadily to deal with the specific challenges that affected us in the first 95% of our evolutionary history, which mainly involved extracellular bacterial infections of wounds. However, with astronomic levels of population expansion and crowded urban dwelling a whole new range of microorganism nemeses have emerged over the past ten millennia and our slowly evolving immune systems are still many steps behind these changes.
Some of these microorganism adversaries may have crossed over from animals to humans after the beginning of organised agriculture (e.g. Tuberculosis coming from pigs) while others may have their origins in avian, bat or other populations (e.g. Coronavirus species).
Whatever their origins, many of these microorganisms are with us to stay, some of them may have direct benefits or neutral effects (e.g. bacteria in the gut biome) but many more are waiting in the wings, replicating rapidly and always likely at some point to churn out a new generation that is highly virulent, transmissible and treatment resistant.
3. Constraints: there are some things natural selection just can’t do
A fundamental principle of evolution is that the process is blind, directionless and there is no overarching plan or design, intelligent or otherwise. Therefore, each new generation of an organism and each new individual mutation is pitted against the prevailing environment of the time and the ‘fittest’ individuals are ‘selected’ to survive and reproduce.
While this process leads to some exquisite phenomena and characteristics that appear to be perfectly ‘designed’ for their environment (e.g. hummingbirds sucking nectar), the process can also be inefficient and lead to problematic vestiges such as the human appendix and our tendency to develop back pain.
The recurrent laryngeal nerve is another often cited example of the limitations and the blind alleys down which the evolutionary process can sometimes take us. The circuitous route taken by this nerve from the base of the brain, down the neck and into the chest, looping around the aortic arch before returning back to the vocal cords serves no clear ‘purpose’. However, now that route is established in mammals, evolution is limited in and has no interest in reversing the obvious ‘design flaw’.
4. Trade-offs: everything in the body has advantages and disadvantages
Certain medical conditions that are ostensibly perceived as illness may also have an adaptive flip side. Classic research by Haldane (Haldane, 1949) on Sickle Cell Disease demonstrated that the heterozygous ‘carrier’ state for this condition conferred protection against Malaria infection in populations living in endemic zones. Thus, while individuals with fully penetrant Sickle Cell disease suffer with the condition and are at risk of medical complications such as sickling crises, the condition may persist because of the protective benefits against Malaria in carrier individuals.
A similar story possibly pertains with Cystic Fibrosis. Among European populations up to 1 in 20 may be heterozygote ‘carriers’ for this autosomal recessive condition, with homozygous sufferers having severe disease trajectories that until recent times led to death in childhood for many sufferers. However, the carrier status may confer protective benefits against Tuberculosis (New Scientist, 2006), in a northern hemisphere analogy for Sickle Cell Disease and Malaria.
5. Reproduction: natural selection maximizes reproduction, not health
Of all the seven principles outlined in this chapter, this one is perhaps the most difficult to accept, as it involves deep reflection on the true nature of evolution. Evolution and natural selection are blind and directionless processes: genes that are associated with traits and characteristics that lead to reproduction will be more highly represented in future generations. Going back to ideas such as the ‘selfish gene’ of Richard Dawkins, evolution has no interest in our comfort or happiness: if a trait makes us deeply unhappy and stressed but increases our reproductive potential then it will be represented in future generations.
“Evolution and natural selection are blind and directionless processes: genes that are associated with traits and characteristics that lead to reproduction will be more highly represented in future generations.”
6. Defensive responses: responses such as pain and anxiety are useful in the face of threats
Evolutionary thinking helps us avoid making the mistake of Viewing Symptoms as Diseases (VSAD) – a potential error in clinical practice and in the thinking of the general public. So just as pain, fever and vomiting may have adaptive and protective benefits in combating illness and are viewed as symptoms of underlying conditions and not as primary disorders in themselves, certain psychiatric symptoms such as anxiety and low mood may have adaptive benefits, especially at milder levels of severity, and should not be regarded as necessarily pathological.
Applying an evolutionary perspective to psychiatric symptoms means first asking the ‘ultimate’ question as to why these symptoms and their associated systems evolved and not just ‘proximate’ questions as to how they are mediated. The evolutionary perspective thus asks why evolution has left us all vulnerable to certain psychiatric symptoms and disorders.
The evolutionary perspective also asks whether or not certain symptoms or response systems may have, (or had, at some stage of our evolutionary history), an adaptive benefit that could improve our reproductive fitness, i.e. our chances of surviving, reproducing and thus seeing our genes appear in the next and future generations.
For example, having a tendency to be anxious and panicky may be protective and advantageous if living in a dangerous environment, especially the much more risky and hostile environments of our ancestors. Randolph Nesse’s ‘smoke alarm’ principle highlights how symptoms such as anxiety and panic may reflect an overly responsive system that leads to an unduly high number of distressing ‘false alarm’ symptoms in an environment where the existential threat is not as significant as in our distant past. Therefore, it can be conceptualised that many false alarms are a ‘price worth paying if it means missing a real fire’. Another evolutionary perspective worth highlighting here again (as outlined in point 5 above) is that symptoms such as anxiety and panic, while distressing and unpleasant, may help our survival chances and, ultimately, our chances of reproducing.
Viewing all symptoms simply as illness and then treating or suppressing them may thus be misguided and lead us to miss broader questions of aetiology for the individual and for us as a species. That said, the evolutionary perspective does not simply ignore or trivialise such symptoms: disrupting dysfunctional response systems and thus alleviating suffering is clearly still required in severe and disabling situations.
The above six evolutionary reasons for illness can be further organised into three pairs. The phenomena of mismatch and the evolutionary race with microorganisms relate to the issue of the slowness of the evolutionary process. The phenomena of constraints and trade-offs highlight that evolution does not produce perfect solutions or outcomes. The last two, relating to symptoms and suffering, highlight how the driving force and ultimate ‘aim’ of genes is to be represented in future generations – evolution has no interest in the health, happiness or wellbeing of the organisms that carry those genes around.
Bonus Principle: Viewing Diseases as Adaptations (VDAA)
While evolutionary thinking may help us reject the error of Viewing Symptoms as Disease (VSAD) it makes us vulnerable to another: Viewing Diseases as Adaptations (VDAA). For example, major mental disorders such as schizophrenia and bipolar affective disorder are unlikely to have any benefits for individual sufferers and should not therefore be viewed as adaptations.
However, applying an evolutionary perspective asks the question why we as a species should continue from generation to generation to remain so vulnerable to conditions such as these.
Regarding mood disorders, the capacity for normal mood variation may well be adaptive, e.g. mildly depressed mood may improve our problem solving through ‘depressive realism’ while depressive avoidance of conflict against a stronger adversary may help us survive to fight another day. On the other end of the normal mood spectrum, brief bursts of energy in the context of opportunity are also likely to be beneficial to us and to our genes. An evolutionary perspective suggests that our universal capacity for mood variation is exaggerated to pathological and dysfunctional levels in those with bipolar affective disorder.
The situation is more complex for schizophrenia, with the most compelling evolutionary theory suggesting that the condition is the high price that approximately 1% of every generation pays for the exquisitely developed language and social cognition skills that the rest of us enjoy.
Conclusions
The seven principles outlined here are essential for a sound understanding of evolution and natural selection and how it impacts on human health and suffering. In following instalments, clinical cases will be presented in a ‘problem-based’ format with learning outcomes and clinical scenarios juxtaposed with evolutionary insights.
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